The present invention relates to a method and a system for determining the capacity of a battery, especially a motor vehicle starter battery.
When using safety-critical, electrical consumers in a motor vehicle, such as brake-by-wire or steer-by-wire systems and electrohydraulic brake systems (EHB systems) or start-stop systems, the energy accumulators supplying these systems must be constantly monitored and, in particular, checked as to their capacity prior to the occurrence of a respective load, so that failures can be recognized and signaled in time, and measures taken to increase the capacity of the energy accumulators, for instance by increasing the charging voltage and/or by switching off consumers, for instance.
Several methods are known for predicting the capacity of starter batteries. In this context, for instance, the determining variables for the battery capacity, such as state of charge and internal battery resistance, are determined, for example, by evaluating the steady-state voltage, and by measuring voltage and current at the start. Alternatively, model-based status monitoring is utilized with ongoing measuring of voltage, current and temperature, and an expected power output of the battery is precalculated on the basis of these quantities. The first method-type has the disadvantage of allowing a recalibration of the state of charge and internal resistance only under particular operating conditions (rest phase or start). At other times, extrapolations are required, leading to errors in the forecasting of the battery capacity, in particular in so-called taxi operation, in which rest phases are rare or very brief. Furthermore, in such methods dynamic changes in the capacity, for example, after demand on the starter battery, are not taken into consideration.
Model-based methods constantly estimate the state of charge and internal resistance during vehicle operation, as well as dynamic voltage drops in the battery, thereby allowing a good prediction of the battery capacity. In practice, however, these methods are very cost and labor intensive, since a battery model covering the entire working range of the battery, as is required in this case, is generally strongly non-linear and includes many parameters to be estimated.
Such a method for determining the state of charge and additional physical variables of a rechargeable electrical accumulator, i.e. a battery, such as state of wear, age, manufacturing tolerances, prior history and charging efficiency, is known from European Patent No. 0 471 698. In this case, process input variables of the energy accumulator are measured and processed in a computing device, whereby, according to the principle of indirect measuring, a predefinable model of non-linear, multiparametrical functions representing the state of charge and the physical quantities to be determined as well as their physical interrelationship, is compared with the measured process input variables, the model parameters being adapted for the subsequent measuring in case of deviations. In this method, a multiparametrical description in the form of the model and a heuristic parametrization are used to determine the state of charge and the physical variables to be determined, the description having more parameters than process input variables, and the under-determined set of solutions of the model being determined first, and the state of charge and the additional physical variables subsequently being determined with the aid of the parametrization as estimation, using characteristics and data of known energy accumulators.
It is an object of the present invention to provide a model which is as simple as possible for estimating a battery performance, in particular the capacity of a battery.
According to the present invention, the capacity of a battery, in particular a motor vehicle starter battery, as it pertains to predefined load current characteristics, especially load current characteristics of specific consumers, is determined by adapting a simple model having a minimum of estimatable parameters, which is applicable to the respective load instance. The model can be implemented as a hardware or software model. Determination of the battery parameters can be carried out during normal vehicle operation and does not rely on extended rest phases or a high-current load such as exists during the start. As a consequence, prompt adaptation of the parameters after changing or recharging the battery is also possible. In addition to static quantities, such as load state and internal resistance, the dynamic voltage drop in the battery immediately after connection of load is also determined, which allows an altogether very precise prediction of the capacity of the battery.
Advantageously, the parameters utilized within the framework of the estimation according to the present invention, i.e., steady-state voltage UR, internal resistance R1 and internal voltage drop UK, are determined with the aid of a monitoring device, especially a Kalman filter, which, on the basis of a measured battery voltage and/or a measured battery temperature and/or a measured battery current, estimates parameters UR, RE and UK by utilizing a model. This model may also be designed as a hardware or software model. It is advantageous if the two models mentioned for describing the battery are essentially compatible.
When comparing voltage response UBatt,pred with a predefinable minimal battery voltage UBatt,limit, the minimal value of the voltage response min(Ubatt,pred) is expediently taken into account. Through such a comparison it is possible to state clearly whether the battery is able to supply a minimal battery voltage UBatt,limit required for the reliable operation of the consumer in question.
Expediently, the capacity of the battery is determined using a formula of the form
SOH=(min(UBatt,pred(t))xe2x88x92Ubatt,limit)/(UBatt,normUBatt,limit),
UBatt,norm being the minimal terminal voltage occurring when a new, fully charged and balanced battery is loaded with the particular consumer at room temperature. SOH=1 for such a battery. The voltage drop could be greater and the SOH value lower, as a result of aging, discharge and prior loading of the battery, and at low temperatures. At SOH=0, the minimal requirement is barely met. It should be pointed out that SOH is the common abbreviation of the English term xe2x80x9cState Of Healthxe2x80x9d of the battery.
Furthermore, it is advantageous to determine the state of charge (SOC) of the battery on the basis of the determined steady-state voltage. This provides an additional static quantity for describing the battery state. Apart from the static quantities, such as state of charge and internal resistance, a very precise prediction can now be made regarding the capacity of the battery, because the dynamic voltage drop in the battery occurring immediately after a connection of a load can also be determined, as described above.